Prior art switching power supplies for electronic systems have included an input converter, at least one output converter and a standby converter. The input converter converts an input alternating current (“AC”) voltage to a relatively high direct current (“DC”) voltage suitable for consumption by the output converters and by the standby converter. Thus the input converter is an AC-DC converter. The input converter usually includes power factor correction circuitry. The output converters convert the high-voltage DC level generated by the input converter into lower-level DC voltages suitable for consumption by the electronic system load. For example, if the load is a server computer, the lower-level DC voltages needed from the output converters are typically on the order of 12VDC, 5VDC or lower. The output converters are DC-DC converters. The standby converter is also a DC-DC converter in that it converts the high-voltage DC level from the input converter into a lower DC voltage level, but the standby converter is configured to supply less demanding loads than are the output converters. The output of the standby converter may be used, for example, to power wake-on-LAN or similar circuitry that must remain active in a server computer when the remainder of the server computer's systems are powered off.
Prior art switching power supplies such as those just described have been capable of operating in only two modes: an online mode and a standby mode. When a prior art power supply operates in its online mode, all of the converters in the power supply are actively switching and the power supply is capable of sourcing full operating current to a load such as a server computer that is fully powered on and operational. When a prior art power supply operates in its standby mode, its output converters are not switching, but its input converter and its standby converter are still both actively switching. The latter mode may be used, for example, when the load itself is in a standby mode requiring only a fraction of the load's full operating power. In such a circumstance, the standby converter in the power supply is capable of sourcing the smaller power demanded by the load without any contribution from the output converters.
Thus, in prior art switching power supplies, the input converter and the standby converter both remain actively switching regardless of whether the power supply is operating in its online mode or in its standby mode.